With the progress of living radical polymerization technology, precise control of molecular weight, molecular end, molecular weight distribution and molecular chain molecular arrangement of a polymer has become possible in recent years. Particularly in controlled living radical polymerization, such as reversible addition fragmentation chain transfer polymerization (RAFT polymerization), atom transfer radical polymerization (ATRP) or nitroxide-mediated radical polymerization (NMP), low-temperature polymerization and strict purification of monomers in anionic polymerization or the like are unnecessary, and there is a merit of a wide application range of monomers. Moreover, there is an advantage that the polymerization can be readily carried out under usual radical polymerization conditions (industrial versatility), and it has become possible to readily prepare copolymers or homopolymers having various structures and functions.
Among them, Non-patent Document 1 describes that various studies of RAFT polymerization, including examination of reaction mechanism, examination of reactivity due to substituents, etc., have been made. Non-patent Document 2 describes that various studies of ATRP have been also made. Non-patent Document 3 describes an attempt to control polymerization of a variety of vinyl-based monomers by means of nitroxide-mediated living free radical polymerization.
By the use of these techniques, fluorine-containing block copolymers can be prepared (Patent Documents 1 to 6).
For example, Patent Document 1 discloses a water and oil repellent block copolymer that is a block copolymer constituted of a fluorine segment and a non-fluorine segment and has a specific contact angle. It describes that such a water and oil repellent block copolymer is obtained by, in the presence of a specific transition metal catalyst, first polymerizing a non-fluorine-based monomer and then polymerizing a fluorine-based monomer.
Patent Document 2 discloses an organic solvent-based coating improver containing a block copolymer that contains, as a polymer unit, a specific fluoroaliphatic group-containing monomer having a repeating structure composed of a methyl group and a repeating structure composed of a difluoromethyl group, and an optical film obtained by using this improver. Patent Document 2 describes that such a block copolymer is obtained by, for example, obtaining a macroinitiator from an acrylic ester or the like through an ATRP process and polymerizing a (meth)acrylate monomer having a fluoroaliphatic group with this macroinitiator by an ATRP process.
Patent Document 3 describes that a block copolymer is obtained by, in the presence of a specific compound having a —C(═S)S— group, polymerizing a first monomer having at least one polymerizable group and then polymerizing a second monomer having at least one polymerizable group and a perfluoroalkyl group.
Patent Document 4 discloses a surface treatment agent containing a fluorine-containing acrylic block copolymer that comprises a specific fluorine-containing acrylic-based segment containing a repeating unit derived from a fluorine-containing monomer and a specific non-fluorine acrylic-based segment containing a repeating unit derived from a non-fluorine monomer. Patent Document 4 describes that the fluorine-containing acrylic block copolymer was obtained by, in the presence of a copper catalyst, first polymerizing a non-fluorine monomer to form a non-fluorine acrylic-based segment and subsequently polymerizing a fluorine-containing monomer with the non-fluorine acrylic-based segment to form a fluorine-containing acrylic-based segment.
Patent Document 5 discloses a block copolymer obtained by copolymerizing, as essential monomer components, a first radical polymerizable unsaturated monomer having a fluorinated alkyl group of a specific number of carbon atoms and a second radical polymerizable unsaturated monomer having a reactive group through living radical polymerization based on ATRP or the like is disclosed. Patent Document 5 describes that the block copolymer is obtained by first subjecting the second radical polymerizable unsaturated monomer to living radical polymerization, then adding the first radical polymerizable unsaturated monomer and further subjecting it to living radical polymerization.
In the Patent Document 6 discloses, as a process for producing a block copolymer by polymerizing a first monomer in the presence of an alkoxyamine catalyst to give a polymer of the first monomer and then polymerizing a second monomer in the presence of an alkoxyamine catalyst and the polymer of the first monomer, a process adopting a fluorine-containing (meth)acrylate monomer as at least one of these monomers.
However, in the case where a (A)n(B)m block copolymer constituted of a fluorine-containing vinyl-based monomer (A) and a non-fluorine vinyl-based monomer (B) is prepared by controlled living radical polymerization, it is necessary that the non-fluorine vinyl-based monomer (B) should be polymerized first to give a polymer (B)m and thereafter the fluorine-containing vinyl-based monomer (A) should be polymerized.
Particularly in the RAFT polymerization, it is often difficult to obtain a homopolymer (A)n of a fluorine-containing vinyl-based monomer, and therefore, a technique is typically adopted which comprises polymerizing a non-fluorine vinyl-based monomer (B) in the first polymerization step and polymerizing a fluorine-containing vinyl-based monomer (A) in the second polymerization step to produce (B)m(A)n.
For example, some examples of RAFT polymerization of a fluorine-containing vinyl-based monomer alone have been reported (see Non-Patent Document 4 and its cited reference). However, the fluorine-containing vinyl-based monomer is limited to a (meth)acrylic ester having a short side-chain fluoroalkyl chain, and such a technique lacks generality as a polymerization technique. Further, a polymerization experiment of 3,3,4,4,5,5,6,6,7,7,8,8,8-tridecafluorooctyl acrylate at 60° C. for 16 hours using 2-cyano-2-propyl dithiobenzoate that is a compound analogous to 4-cyanopentanoic acid dithiobenzoate used in the Non-Patent Document 4 (this experiment was carried out by the present inventors) resulted in an unsatisfactory degree of polymerization of 19%.
When a fluorine-containing block copolymer containing, as a non-fluorine vinyl-based monomer, stearyl acrylate or stearyl methacrylate having a long side chain, or styrene is produced, polymerization of the non-fluorine vinyl-based monomer in the first polymerization step proceeds, but in the second polymerization step for polymerizing a fluorine-containing vinyl-based monomer, living property is lost, and a homopolymer (A)n of the fluorine-containing vinyl-based monomer (A) is produced in a large amount.
Moreover, also in the case where the side chain of the non-fluorine vinyl-based monomer (B) is short, if the weight of the fluorine-containing vinyl-based monomer (A) fed in the second polymerization step is relatively large as compared with the weight of (B)m produced in the first polymerization step (e.g., (B)m:(A)=20 wt %:80 wt %) as in the case of production of a fluorine-containing block copolymer having a high content of the fluorine-containing vinyl-based monomer (A), a homopolymer (A)n of the fluorine-containing vinyl-based monomer (A) is often produced in the second polymerization step.